Interaction between mutagenic effects of strong homogeneous magnetic fields and fission neutron-plus-gamma radiation was investigated, using the sex-linked, recessive, lethal test in Drosophila melanogaster. Drosophila males were exposed chronically, for seven days, to a 37,000-G homogeneous magnetic field and/or 3 doses of neutron-plus gamma radiation. Mutations in spermatozoa, spermatids, and spermatocytes were scored. There was no evidence of interaction between the effects of the two types of exposure in causing genetic damage in any of the three cell types. Dose-response relations for the radiation doses were linear for spermatozoa and spermatids for all three doses and for spermatocytes, up to 300 rads. Spermatozoa appear to be most sensitive to neutron-radiation-induced, recessive lethal mutations.